Oscillators are used to supply a reference frequency in electronic circuits of, for example, various kinds of home appliances, communication apparatuses, and information processing systems. The oscillators can be classified into oscillators using a signal delay caused by a circuit, such as ring oscillators or CR oscillators, and oscillators using a resonator. It has generally been known that the latter has higher frequency accuracy than the former.
The resonators can be classified into resonators using electromagnetic resonance, such as LC resonators, and resonators using mechanical resonance, such as piezoelectric resonators or MEMS resonators. It has generally been known that the latter has higher frequency accuracy than the former.
In the related art, piezo-resonators, such as quartz resonators or surface acoustic wave resonators, have been used in order to obtain high frequency accuracy. In this case, it is difficult to incorporate the piezo-resonator with other electronic circuits including, for example, an amplifying circuit. A ring oscillator or an LC oscillator has been used in order to improve the density of integration between the oscillator and other electronic circuits. However, in this case, it is difficult to obtain high frequency accuracy.
An integrated oscillating circuit has been proposed which is provided with a variable capacitance element in order to obtain high-accuracy oscillating frequency.